I agree, the distance is just too small. and also it just provides current, so you probably need more than just the battery pack. Other than that I could see interesting applications outside dive lights.
I worked on one once. It's harder than it looks. The coupling is so poor you have to jump through some serious hoops to get any usable power. Notice how the coils are carefully and neatly wound, and bonded.
Bigger coils would help, but these are already fairly good sized for some flashlights. What else would help is if you could get he proper magnetic core halves to make it a gapped core rather than full air core.
If I were interested in toying around, I would buy these and use a DC/DC convertor to get the output I wanted. It would be a much easier job than a scratch build.
Perhaps you could get it to work but I've never been a big fan of sealed units. Corrosive off-gassing and humidity trapped in the container (you could purge as much of the humidity as possible no doubt). It would be helpful if you could have a way of opening it up from time to time for servicing, without destroying it.